1 Department of Virology, Faculty of Medicine, University of the Ryukyus, Okinawa, Department of Bacteriology, Institute of Tropical Medicine, Nagasaki University, Nagasaki, and Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, and Department of Molecular Virology, Graduate School of Medicine, Tokyo Medical and Dental University, Tokyo, Japan; Department of Pediatrics, Stanford University School of Medicine, Stanford, California; and Merck Research Laboratories, Merck & Company, Inc., West Point, Pennsylvania